The Plasticity of Pancreatic Stellate Cells Could Be Involved in the Control of the Mechanisms that Govern the Neogenesis Process in the Pancreas Gland

Immunocytochemistry is classically defined as a procedure to detect antigens in cellular contexts using antibodies. However, over the years many aspects of this procedure have evolved within a plethora of experimental setups. There are different ways to prepare a given specimen, different kinds of antibodies to apply, different techniques for imaging, and different methods of analyzing the data. In this book, various ways of performing each individual step of immunocytochemistry in different cellular contexts are exemplified and discussed. Applications of Immunocytochemistry offers technical and background information on different steps of immunocytochemistry and presents the application of this technique and its adaptations in cell lines, neural tissue, pancreatic tissue, sputum cells, sperm cells, preimplantation embryo, arabidopsis, fish gonads, and Leishmania.


Introduction
Mammalian pancreas is a gland that plays an important role in the regulation of energy balance and nutrition. Through the synthesis and release of protein digestive enzymes and hormones, which are involved in the absorption, it uses and stores the digested nutrients. This gland divided into two compartments with exocrine and endocrine functions, together with the stroma surrounding the pancreatic parenchyma, plays important roles in the homeostasis of the body. Moreover, they are involved in the maintenance of the function of the organ, including the regenerative process observed after injury of the pancreatic tissue. However, to understand this relationship, it is necessary to understand the embryological mechanisms that control the development of the pancreatic tissue. This embryological pathway begins from the precursor cells located in the endoderm, which is able to promote the pancreatic morphogenesis after responding to specific external and internal signals. Therefore, knowledge of the different networks created by neighbouring embryonic tissues will be essential for understanding the complexity of this morphogenetic process.
The organogenesis process of the pancreas gland is originated from stem cells located in the endoderm, which have the capacity to promote the development of the exocrine and endocrine compartments, identified in the adult gland from mammals. This phenomenon follows a specific gene network activity which is regulated by specific transcription factors (Jensen J, 2004). This complex process can be summarized into three steps identified by different investigators. The first step is accomplished through the action of specific signals that are originated from the mesoderm (Sander M and German MS, 1997). In the second step, the primitive endocrine cells, which are scattered throughout the undifferentiated epithelium, proliferate and promote the primitive islets cells located in the surrounding mesenchyma. Moreover, the mesenchymals signals are important to promote the development of islet cells and increase the number of beta cells at the end of the process. All these signals also promote vascularization (Kim SK and Hebrok M, 2001;Scharfmann R 2000;Reusens B and Remacle C;. In the last step, the gland is remodeled into two functional compartments (Habener JF et al. 2005). In the adult pancreas, these two compartments exhibit different physiological roles. In addition, they have an important relationship and cellular interaction.
The pancreas like other tissues is considered like a dynamic organ, able to adapt to different physiological situations, such as diabetes, obesity or in gestation. This dynamic adaptation is based on the regulation of the beta cell mass in order to maintain glucose homeostasis. There are different mechanisms that control this process, which include: apoptosis, necrosis, hypertrophy, hyperplasia and neogenesis. However, little is known about some of these processes, and in particular, the cells which are involved. In the case of the neogenesis process, many studies supported the idea that it occurs via cells which are located in, or which are associated with, the ductal epithelium of the exocrine compartment of the pancreas. One of the approaches used for investigating this hypothesis is the application of the immunocytochemical and immunohistochemical techniques. These techniques are important because they help to identify the cell population involved in the process without losing the architecture of the tissue. Moreover, they are important tools for the phenotyping of the cell population when isolated from the tissue and checked while maintained in vitro.

Historical perspective of stellate cells
In 1876 Karl von Kupffer described for the first time a new population of cells in the liver called "sternzellen " or stellate cells, due to their stellate appearance. These cells located in the space of Disse had cytoplasmatic inclusion bodies indicating to have a phagocytic function. Initially, Kupffer classified them into the "Waldeyer's perivasculare Bindgewbszellen" or reticulo-endothelial system. However, this author changed opinion and the cells were considered phagocytes and were referred to as "special endothelial cells of the sinusoids" (Kupffer C 1876). However, it was not until the beginning of the 20 th century when Zimmerman described them as dendritic perisinusoidal cells surrounded by reticular fibers and named them hepatic pericytes. Later, the Japanese Anatomist Dr. Ito described a new cell population in the liver, which were located in the perisinusoidal space and contained abundant amounts of fat droplets in their cytoplasm. These cells, known as "Ito-cells" are able to store and deliver vitamin A and other liposoluble vitamins. Moreover, they are involved in the regulation of sinusoidal tone, local blood supply, and tissue repair and fibrosis. The cell presents several thick cytoplasmatic processes which are protuded directly from the perikaryon (primary process) and extended onto the outer surface of the sinusoidal entohelial cells (Ito T et al. 1951). In summary, these cells have received other names, such as: fat storing cells, pericytes, parasinusoidal, and lipocytes. Several studies demonstrated that all these cell populations shared most of their cellular and physiological characteristics and seemed to correspond to the same population. For that reason, and in order to avoid confusion, in 1996 the international community of investigators unified the nomenclature and defined these cells as a "Stellate cells" (no authors listed, 1996). Soon after, Kent and Popper demonstrated that the stellate cells were linked to the pathogenesis of hepatic fibrosis (Hirosawa K and Yamada E, 1973). This important finding promoted the identification of this cell type in extrahepatic organs (pancreas, spleen, adrenal, ductus efferent and uterus) in rodent and humans (Geerts et al., 2001).
In addition, the presence of these cells in a wide variety of species, ranging from lampreys (primitive fish) to humans and in all major tissues, indicated their importance in the development of the different organs (Wake K 1987).

Stellate cells in pancreatic tissue: historical perspective
Vitamin A storing cells were first described in the pancreas by Watari, et al., in 1982, using fluorescence andelectron microscopy. In 1990, Ikejiri, et al., confirmed the previous results and also showed the presence of vitamin A as a autofluorescence stained in normal pancreatic sections from rats and humans. In 1997, Saotome, et al., described the presence of the myofibroblast-like cells in human pancreas, and their involvement in the extracellular matrix remodeling during the fibrosis process. However, these independent observations had not been realized to be related until 1998, when Bachem, et al., and Apte, et al., defined these two populations of cells as pancreatic stellate cells, in two different stages of activation (Quiescent and Active).

The embryological origin
The embryological origin of the stellate cells is unclear. Importantly, there are few studies conducted to resolve this dilemma. Most of them have been described in the liver. For that reason, different observations of these cells in liver have been extrapolated to other organs including the pancreas. However, numerous theories on the linage of these cells have been presented. The hepatic stellate cells (HSC) are proposed to be derived from mesenchymal cells that separate the pericardial and peritoneal cavities of the embryo (Morita M et al. 1998;Naito N and Wisse E 1977). However, the specific microfilaments identified in their cytoplasm and morphology, resembling the astrocyte cells from astroglia in the Central Nervous System, could also be indicating a neural-ectodermal origin (Niki T et al. 1999;Friedman SL 2000). This last observation was difficult to reconcile with the mesenchymal origin described before. Recently, the identification in bone marrow of fibroblast /myofibroblast cells, which share some HSC characteristics, suggests that stellate cells could be derived from hematopoietic stem cells (Susking DL and Muench MO, 2004;Baba S et al. 2004;Ogawa M et al 2006). In conclusion, new experimental designs are required in order to understand the embryological origin of these cells. Moreover, the possibility to use the lineage-specific promoters to drive the transgene expression could contribute to the clarification of this problem and enable the understanding of the biology of these cells.

Biology of pancreatic stellate cells
Pancreatic stellate cells (PSC) are located in different spaces: periacinar, perivascular and periductal of the exocrine compartment of the pancreas. They represent approximately 4% of the total cells of the gland. The cells are closely in contact with acinar, endothelial and www.intechopen.com ductal cells and establish a strict cellular communication between them through long processes containing numerous filaments and microtubules. These cells play an important role in the pancreatic pathology of the exocrine compartment of the pancreas, such as chronic pancreatitis and pancreatic cancer. In all these injury processes, PSC and HSC have shown an important phenotype transformation to a so-called activated form. In this state, the cells are able to produce large amounts of extracellular matrix proteins (EMC), fibronectin and laminin resulting in the extensive fibrosis. In this stage, the cells showed: a typical characteristic spindle -shaped, absence of the retinol in the cytoplas, the increment of the myofilaments, as in the GFAP and vimentin, as well as the presence of the new myofilament ( -SMA). Moreover, the production of multiple factors with a paracrine, autocrine and chemoattractant actions can be detected (Jasper, R 2004, Morini S et al. 2005Omary MB et al. 2007; (Fig.1 A,B). In contrast, when the cell are in the quiescent form, they present: abundant droplets of vitamin A in the cytoplasm, are less positive for desmin, vimentin, nestin and GFAP intermediate filaments, and the cytoplasmatic processes are not observed. In addition, a non-proliferative state is observed in the cells (Pinzani, M. 1995;Apte MV et al. 2003). The transitional stage of the cells was observed and the cells share some of the ultrastructural and functional characteristic for these two differentiated stages described previously.
The mechanism implicated in this transformation process is not determined yet. In vivo, different signal transduction pathways have been described and all, including infiltrating leucocytes and damaged acinar cells, are able to initiate and maintain the activated phenotype. However, most of the information about the activation mechanism has derived from in vitro studies of rodent PSC maintained in culture. These cultures, initially express the molecular markers of the quiescent cells and it is easy to observe the presence of the cytoplasmic lipid droplets by oil red stain (Apte MVet al., 1988, Mato E et al 2009. However, in a short amount of time, most of the cells in the culture showed a proliferative phenotype with -SMA and ECM protein expression. These molecules are associated with the activated phenotype . Several authors have associated this phenomenon to in vitro changes of Rho-ROCK pathways regulated by the actin cytoskeleton . PI 3-kinase activity is required for PDGF-stimulated PSC migration, but not cell proliferation (McCarroll JA et al. 2004). Moreover, the role of the enzymes involved in the mitogen-activated protein kinase (MAPK) family have been described : Jun N-terminal kinase JNK and p38, which are involved in the transcriptional control and PSC activation, and are mediators of signals induced by pro-inflammatory cytokines and cellular stressors ). On the other hand, ligands of the nuclear receptor PPAR (peroxisome proliferator-activated receptor ) such as 15-deoxy-Δ12,14-prostaglandin J 2 and troglitazone (an antidiabetic drug of the thiazolidinedione group) stimulate maintenance of a quiescent PSC phenotype in vitro have been described . In summary, despite that several intracellular mediators involved in the control of the PSC activation and desactivation have been identified, most of them are unknown.
Furthermore, some authors have documented a significant increment of the PSC in the regenerative areas of the pancreas after suffering an acute pancreatitis, induced in rodent. These observations, plus the identification of the PSC positive for nestin marker, support the idea that this population could be involved in the pancreatic regeneration process (Zimmermann A et al. 2002, Ishiwata T et al 2006.

Fig. 1. A. Transmission electron micrographs of activated pancreatic stellate cells in culture.
The arrow show abundant collagenous fibers compatible with collagenous type I. B. RT-PCR expression involved in the EMC remodeling (Mato E. et al., unpublished data)

Pancreatic progenitor cell: historical perspective
One of the important reasons to find progenitor cells in the pancreas is to cure Diabetes Mellitus. This metabolic disorder is a common and serious disease in our society and is the most rapidly growing chronic disease of our time. It has become an epidemic that affects millions of people around the world. For that reason, there has been an increasing in interest scientific community to identify the cell populations with stem or progenitor properties in the pancreatic tissue. This finding could represent a significant therapeutic advance in this disease.
The first description of stem and progenitors cells in adult tissue was in bone marrow and the nervous system (Weissman IL 2000;Fuchs E and Segre JA 2000). Although it is accepted that similar cells can exist in the other adult tissues and organs, they are not always easy to find. One of the reasons for limited number of studies on these cells relates to the fact that they do not have specific biological markers. Thus, finding of progenitor cells in the pancreas is a challenge. There is some evidence in the pancreas that progenitor cells exist in the neogenesis process, which can be induced by cellophane wrapping of the pancreas (Rosenberg L et al. 1998), partial pancreatectomy (Bonner-Weir S et al. 1993), streptozotocininduced diabetes (Fernandes A et al. 1997), and also during pregnancy (Bonner-Weir S 2000). Some authors, Rosenberg in 1998 andRafaeloff in 1997, have only associated this phenomenon with gene (Reg) and proteins (islet neogenesis, INGAP) which are expressed during the process, but not with progenitors cells. However, cell participation is possible.
Research has been launched to investigate the process of neogenesis and the cells that may be involved in this mechanism. Understanding this process will be the key since it will allow us to restore the function of the gland lost during the illness.

Ductal cells
Most of the studies favor the pancreatic duct as a potential source of progenitor cells in adult pancreas (Rosenberg L 1998;Bonner-Weir S 2000). These studies are based on the information about the important role the primitive ductal epithelium has during the pancreas embryogenesis of the pancreas as a source for the islet development (Madsen OD et al. 1996, Sander M andGerman MS, 1997). Moreover, Gu and coworkers described the presence of endocrine cells within the adult ductal system (Gu D and Sarvetnick N 1993) and also identified beta cells associated with the human ductals (Bouwens and Pipeleers, 1998). Finally, the ability of ductal cells to expand in vitro and to form insulin-producing islet-like structures has also been demonstrated (Bonner-Weir S et al. 2000;Ramiya VK et al. 2000).

Pancreatic islet as a cellular source
Another interesting hypothesis was to propose the pancreatic islet as a progenitor cell source, based on the analysis of islet regeneration in mouse pancreas models after the administration of streptozotocin. The results showed the presence of the insulin-producing cells following the injury into the adult islets. This study suggested the existence of the two types of progenitor cells, one of them expressed Glut-2 and the other coexpressed insulin and somatostatin (Guz Y et al. 2001).
Nestin-positive cells, neurogenin-3 positive cells and hormone-negative immature cells, with proliferative capacity in vitro has been found in rats and human islets. This supports the idea of the existence of the multipotential cells in the islet (Kodama S et al 2005). However, their participation in islet regeneration and neogenesis in vivo has not yet been demonstrated (Zulewski H et al. 2001). Despite the explosion in the number of in vitro studies that describe different types of cells with progenitor capacity within the island, there is also some critical work demonstrating that the reactivation of genes required for endocrine cell development, such as neurogenin 3, are not implicated directly in the regeneration of pancreatic tissue after pancreatectomy (Lee CS et al. 2006).
Cells with the capacity to be differentiated not only in the lineage of endocrine cells, but also in other cellular lineages, such as exocrine and glials cells, have been identified (Seaberg RM et al 2004). These progenitors could be of different origins (ductal cells or cell located inside the islets). These cells showed different molecular markers, such as "the hepatocyte growth factor receptor", c-Met. This receptor tyrosine kinase plays an important role in tumour growth by activating mitogenic signaling pathways (Seaberg RM et al 2004;Suzuki A et al., 2004).
Other authors identified cells presenting a differentiated morphology and named them "small cells". Although these cells are positive for several pancreatic markers (PDX-1, sinaptoficin, insulin, glucagon, somatostatin, pancreatic polypeptide), they also expressed www.intechopen.com markers of undifferentiated cells, such as: alfa-fetoprotein and Bcl-2. Surprisingly, these cells were negative for nestin and cytokeratin 19, indicators of pluripotency and ductal origin. Functional analysis showed that they have the capacity to present a glucose response, but they did not respond to secretatgogues, such as IBMX (Petropavlovkaia M and RosenbergL , 2002).

Hematoipoietic stem cells as a progenitor cells in pancreas
Hematoipoietic stem cells have been proposed, as a new progenitor source in pancreas. In 2002, this hypothesis was formulated by Lerner, et al., who identified a population defined as Side Population, or SP, from a bone marrow origin. This SP cell population, described for the first time by Goodel MA, et al., corresponded to a small subpopulation of cells with an enriched stem cell activity and showed a "low" Hoechst 33342 dye staining pattern. Subsequent studies attributed this SP phenotype to the expression of stem cell markers sucha as MDR1 and Nestin, and also co-expressed ABCG2, an ATP-binding cassette (ABC) transporter (Zhou S, 2001). ABCG2 gene is expressed in several rodent tissues, such as in the intestine, kidney and testes (Tanaka, Y 2005). The precise physiological function of these transporters in progenitor and differentiated cells is unknown and it has been postulated that they confer protection against a number of xenobiotics, thus maintaining the regenerative capacity of the tissue (Leslie, E.M, 2005). The identification and isolation of ABCG2 positive cells in pancreatic tissue may be a new potential source of adult multipotential stem/progenitor cells, useful for the production of islet tissue for transplantation into diabetic subjects (Fetsch, PA, 2006). The presence of these cells in pancreas tissue is controversial.

Epithelia Mesenchyma Transition (EMT)
Finally the concept of Epithelia-Mesenchymal transition or EMT has been described during the regeneration endocrine pancreas and in the cancer development. The EMT could permit that adult cells can be differentiated into the fibroblastic-like cells as a step of transition to other cellular lineage. Recently this process has been linked with the maintenance of stem cell phenotype. However, the molecular mechanism to control the EMT process remained to be demonstrated (Gershengorn MC et al. 2004;Bonner-Weir S et al. 2004).
An explosion of publications in the last decade tried to discover what type and where the progenitors cells are localised in the pancreatic tissue. We can conclude that the number of progenitor cell types in the pancreas may not be too limited to the cells already described. It is possible that the pancreas may contain an unidentified cell population at rest, as described in oval cells in the liver, capable of initiating their proliferation during the process of neogenesis. This opens the opportunity to explore new cell populations that form the pancreatic parenchyma.

Immunocytochemical investigation of the role of pancreatic stellate cell as progenitor cell
The plasticity of the stellate cells phenotype during tissue injury is a proven fact and may indicate that these cells can be presented in progenitor cell features. These findings suggested a novel aspect of the stellate cell biology must be necessary investigated. www.intechopen.com The first marker identified in HSC was nestin. Nestin, a marker for neural stem cells, was identified in HSC during the transition from the quiescent to the activated phenotype in cells maintained in culture, but no association with a progenitor role was suggested by the authors (Niki T et al. 1999). Later, other markers were identified in the HSC: CD133 (prominin-1), a glycoprotein also known in humans and rodents as a Prominin 1 (PROM1), and expressed in the adult and embryonic stem cell and Oct4 (octamer-binding transcription factor 4), also known as POU5F1 (POU domain, class 5, transcription factor 1), protein involved in the self-renewal of undifferentiated embryonic stem cells (Mizrak D et al. 2008;Niwa H et al. 2000). These two markers were able to maintain an undifferentiated phenotype without losing the ability participates in liver regeneration . Finally, the HSC were able to be differentiated into endothelial or hepatocyte-like cells (Kordes eC t al. 2007;Kubota H et al 2007). Following these findings an increasing number of papers about this topic were published.
The existence and lineage of progenitor cells in the pancreas, as well as their origin and location, is a topic of debate and, although several hypotheses had been proposed, it is not yet proven. Moreover, the possibility that the PSC can act as a progenitor cell is not clear.
Nevertheless, it is also important to remark that PSCand hepatic stellate cells are identical, have a common origin and both share transcriptional level, exhibiting organ-specific variations of the common transcriptional phenotype and (Bucholz M et al 2005; Omary MB et al 2007). This scenario suggests that the progenitor role for PSC could be a reality. In 2002, nestinpositive cells were identified in normal adult rat pancreas and during its regeneration. Interestingly, most of these cells presented the morphology characteristic of stellate cells. Nestin, in pancreas as in liver, was confirmed as a main marker of stellate cell activation. Other roles, including the marker of progenitor cells, were not confirmed (Lardon J 2002).
The question that needs to be addressed is whether PSC, after overexpressing some specific pancreatic transcription factors, such as Pdx1 or NeuroD1, have the ability to present the transdifferentiation process, which permits conversion into insulin-producing beta cells.
One approach to conduct these studies and broaden the possibility of unraveling the mechanisms that control self-renewal, is to explore the cell roles after their isolation and establishment of the cell culture. The first description of the stellate isolation from tissue was in 1977 by Galamos JT. The study was characterized by growth mesenchymal cells derived from liver tissue, which have probably been derived from stellate cell (Galamos JT et al. 1977). Later, density gradient centrifugation was used after in situ digestion of the tissue, based on their buoyancy attributable to intracellular vitamin A. The density gradient separation method remains the most widely used approach for stellate cell isolation, but criticism of this method favours the isolation of quiescent cells, which are rich in vitamin A (Friedman SK, 2008). Later, transgenic and knockout mouse models have been developed for the isolation following the standard method of murine stellate cells or for performing in situ analysis with specific stellate markers. However, one limitation of the technique is the large number of animals needed to obtain an adequate cell yield (Henderson NC et al. 2006;Kalinichenko Vet al. 2003). To solve this problem, stable cell lines obtained from human and mouse model would be an important advantage for many investigators in order to study stellate cell biology. Several methods have been described to establish from HSC cultures and pancreas cell lines, such as: long-term culture, transfection with simian virus 40 (SV40) T antigen, or ectopic expression of telomerase (Vogel S et al. 200;Murakami K et al 1995; www.intechopen.com Apte MV et al. 1998;Kruse ML 2001;Sparmann G 2004;Jesnowski R et al 1999;Löhr M et al 2001). The disadvantage of the cell lines is that they differ somewhat in their state of activation or in transcription expression and the results obtained must be validated in the in vivo model. Finally, the description of the cryoperservation technique for freezing primary stellate cell lines is an important advance for sharing the cells between different laboratories (Neyzen S et al. 2006).
In this context our group initiated a new research field, focusing in the identification of progenitor cell in pancreas tissue through ABCG2 transporter as a progenitor cell marker. This marker was identified as a molecular determinant of the Side-Population (SP) phenotype. However, there is no information about its expression on the pancreatic cells. Recently, overexpression of the breast cancer-resistance half-transporter protein (BCRP1) was found to be responsible for the occurrence of mitoxantrone resistance in a number of cell lines (Doyle LA et al, 1998;Miyake K et al, 1999;Litman T et al, 2000). Based on in these findings, we isolated a mitoxantrone-resistant cells population from pancreata of lactating rats by mitoxantrone selection through the ABCG2 transpoter ( Fig. 2 A, B, C).  The existence of a fine balance between proliferation and differentiation process is accepted by the research community. This balance promotes the differentiation from adult stem cell to postmitotic cells through decreasing or increasing the ratio of proliferation, permitting the maintenance of the stem cell population in adult tissues (Soria B, 2001). The observation of the behavior of mitoxantrone resistant cells in culture was interesting. The results indicated C www.intechopen.com that, while the cells with fibrobastoide appearance have showed a rapid and constant growth after clustering formation, they modified their behavior showing a significant reduction in their growth, without stopping completely (Fig.3, B). The results suggested the ability of the cell to be reprogrammed.
Finally the immunocharacterization of these cell cultures in monolayer and cellular cluster showed a stellate phenotype, characterised by vitamin A uptake (oil red staining) and steallate markers presence (Fig. 4 A, B). Moreover, they share markers of the adult stem cells, such as: ABCG2, Nestin, Thy1.1, and N-CAM. The latter marker participates in signal transduction and in cell type segregation as a mediator of cellular junctions during organogenesis (Esni F et al. 1999) (Fig. 5).

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Little it is known about the role of Fibroblast growth factor and their receptor in stellate cells. FGF belongs to a large family of molecules that retain a high homology at the genetic level. These growth factors induce pleiotropic responses, causing effects in both embryonic development and in adult tissue (Steiling H and Werner S, 2003). Their actions are mediated by four receptors of the tyrosine kinase membrane and present different isoforms (b and c) by splicing (Itoh N and Ornitz DM, 2004). Fibroblast growth factors receptors (FGFR) have been detected over time during the development of the pancreas. In addition, their ligands, such as Fibroblast growth factor: 1, 7, 9, 10, 11, 18 (Dichmann DS et al., 2003), and the subtype of the FGFR 2, called FGFR2b, seem to have a key role in the exocrine development (Miralles F, et al. 1999). Recently, FGF7 and FGF10 have been involved in maintaining the cells in an undifferentiated stage and controlling the self-renewal of the pancreatic precursors (Elghari L et al. 2002;Norgaard GA et al, 2003). The positive gene expression for FGFR2IIIb, FGFRIII2c, FGFR1, and their specific ligands (FGF 1,7,and 10), were showed for the first time in our cell cultures (Fig. 6, Mato et al. unpublished data).  (Mato E. , unpublished data) This finding may suggest that FGFR and their ligand are involved in epithelialmesenchymal communication of PSC and, in addition, the autocrine effect allows the maintenance of its cell population in the pancreatic tissue. On the other hand, pancreatic stellate cell do not express endocrine genes. However, during cell expansion, a spontaneous cell differentiation occurs and these cells showed a weak expression of PDX-1 in to the nucleus and the cytoplasm of the cells (Fig. 7 A, B). This gene, also known as (insulin promoter factor-1, islet/duodenum homeobox-1, somatostatin transactivating factor-1, or insulin upstream factor-1 and glucose-sensitive factor), plays a key transcription factor in the endocrine differentiation pathway and is also essential for differentiation of endocrine cells in the gastric antrum. The results suggest a transdifferentiation process. However, the molecular mechanisms of this process are unknown. In additon, few studies are investigating the effect of culture medium and additional protein components on the viability and maturation of the cells (Royer PJ et al 2006). Our results underscore the www.intechopen.com importance of defining culture medium composition in experimental procedures, in order to identify new soluble factors involved in the processes of cellular transdifferentiation. Identifying instructive signals that induce differentiation during organogenesis will be important to determine how such signalling networks are established and how they elicit multiple signalling responses in endodermal cells to activate appropriate genetic programs (Ratineau C et al 2003). Several signalling molecules have been implicated in induction of specific endodermal cell types. However, few of these factors have been examined in adult pancreatic tissue (Sttaford D et al 2006). One of these factors is GLP-1, secreted from the Lcells of the distal ileum and colon. This substance has been suggested to play an important role in increasing beta cell mass by inducing the neogenesis or transdifferentiation through the expression of Pdx-1 in ductal or islets cells (Yue F et al. 2006;Abraham EJ et al 2002;Hui H et al 2001).
Also, matrigel secreted by Engelbreth-Holm-Swarm (EHS) mouse sarcoma cells, is a gelatinous protein mixture that provides a semisolid medium that resembles the complex extracellular environment found in many tissues and is used as a substrate for threedimensional cell culture. The addition of exendin-4 (analog to GLP-1) and matrigel to our cellular model was needed to proceed to the differentiated stages and permit detection of insulin, IAPP, glucagon, GLUT2 and the convertases PC1/3 and PC2 expression (Fig. 8 A,  B). In contrast, expression of the transcription factor p48 and other exocrine genes, such as amylase, were not detected. Interestingly enough was the observation of the cytokeratin 19 (CK19) expression. These intermediary filaments present in cells of the epithelial origin, such as ductal cells, indicate that the cell could be involved in the mechanism to control the mesenchymal-epithelial transition (MET). This phenomenon consists of a promising source of cells for replacement therapies, but can also be involved in the carcinogenesis process (Mato E et al. 2009  The molecular mechanisms and the receptors involved in EMT process are not indentified yet. Most of the evidence suggests that integrin could play an important role. On the other hand, the basement Membrane Matrix is an effective culture medium for the attachment and differentiation of both normal and transformed anchorage dependent on epithelioid and other cell types. The use of these three-dimensional culture systems may be particularly relevant to such efforts by recapitulating a more physiological microenvironment (Han YP et al. 2004;Phillips PA et al. 2003;George PC 2005). During the matrigel growth, substantial ultrastructural changes in the cells were observed. The cells presented a smaller and more homogenous cell size with round nuclei and electron-dense homogenous chromatin, a significant increase in the number of mitochondria, lipid droplets in the cytoplasm and abundant electron-dense granules were also observed. In contrast to the cellular cluster growth in a normal condition medium, the quiescent stellate cells had a high presence of fibers compatible with collagen fibers (Fig. 9 A).

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Gene expression and ultrastrucutral changes detected in the cell culture growth support the idea of the ability of cells to release insulin into the medium. In this scenario, insulin secretions of several sets of cell clusters were measured by static incubation at low (2.8mM) and high (20mM) levels of glucose. Eventhough, insulin levels detected in the cell clusters were lower compared to mouse islets, an increase of 44% was detected after stimulating cellular clusters with high level of glucose. (Fig. 9 B). However, future experiments will have to demonstrate that the secretion of insulin is not only constitutive (Kuliawat R et al. 1994). Furthermore, the expression of specific markers of stellate cells remained after maintaining the cell in matrigel condition. These results may indicate the differentiation process has not been fully completed and the cells still maintained characteristics of stellate cells (Fig. 8).
An interesting strategy in order to investigate the biology of these cells is the use of proteomic approaches, since it is a useful tool for displaying protein expression patterns in the cell. For that reason, this approach has been used in active as well as quiescent stellate cells. (Kawada N et al. 2001;Pauki JA et al. 2011 (a); Paulo JA et al. 2001 (b);Wehr AYet al. 2011). In this context, the proteomic study of our cellular culture secretome was preformed.
The results showed that some of these proteins have potentially great influence on the physiology of the stellate cells themselves and/or on neighbouring cells, indicating a paracrina and /or autocrine action. Moreover, we have identified some novel factors that were clustered in the differentiation/development-related proteins, such as AHNAK, Gap43, and DIXDC1 (unpublished data from Mato E et al ). However, further experiments are required to investigate the interaction within these different genes.
In summary: The pancreatic stellate cells is a fascinating nonendocrine cellular model that could represent a new source of cells involved in regenerative medicine of the pancreas in the future. However, more studies are needed to understand the molecular mechanisms that control their cellular plasticity. Certainly, the use of imunocytochemical and immunohistochemical techniques, complemented with cell -tracking methods, will be important tools to unravel the role of these cells during the tissular regeneration process both in the pancreas and in the liver.

Acknowledgment
The authors thank Scientific and Technical Services of the University of Barcelona (SCT-UB, Campus Casanova) for technical support with electron microscopy, and Julie Shouer-Leventhal for editorial assistance. This work was partially supported by the Spanish FIS grant from the Ministry of Health -FIS PI020881, by Sardà Farriol Research Program and CIBER-BBN and CIBERDEM are ISCIII (Instituto de Salud Carlos III) projects.